Many different types of administration sets are known for delivery of fluid to a patient. In a gravity-feed administration set, an elevated reservoir of the fluid to be administered provides a source for a downhill flow of fluid through the administration set. Resistance to the flow of the fluid is caused by a variety of natural sources; typically, most of the sources maintain a substantially constant resistance so that by adding an adjustable resistance to the flow, the operator of the administration set can control overall resistance to flow and thereby also control the rate of fluid flow to the patient.
In the past, to provide for a source of adjustable resistance it was common practice to place a clamp on the tubing leading the flow of the fluid from the reservoir to the patient. By manually varying the amount the clamp pinched the tubing, the resistance to flow was controlled. More recently, devices have been developed which automatically adjust the degree of pinching provided by the clamp in response to the sensing of the flow rate of the fluid. These devices are commonly called flow controllers, and they represent a significant improvement over manual adjustment of the clamp since the controller constantly monitors the flow and makes real-time adjustments to the flow resistance provided by the clamp if such adjustment is required. Real-time adjustment is a valuable feature since overall resistance to flow of the fluid may change during the course of delivery. For example, movement by the patient may cause the end of the catheter to slightly change its position in the vein of the patient. Such a change in position may cause an otherwise static source of flow resistance to suddenly change in value. Because of their ability to provide dynamic control of the resistance provided by the clamp, the flow controller compensates for changes in resistance to flow caused by movement of the patient and the like.
In addition to dynamic changes in the overall resistance to fluid flow in the administration set, changes may occur in the set which create fluid flow commonly referred to as abnormal flow. Abnormal flow is any flow rate which cannot be satisfactorily corrected by the normal real-time adjustment of the clamp of the flow controller. For example, dynamic changes in the flow characteristics of the administration set may result in the discrete drops falling in the drop chamber changing to a continuous stream of fluid. Such a continuous stream is particularly characteristic of a flow rate which is much too great for small-sized orifices in drop chambers. This condition is difficult to detect with the high degree of reliability required for safety considerations.
In contrast to streaming, abnormal flow may also be characterized by a stoppage of the flow. Such a stoppage of flow may be caused by a temporary and/or partial occlusion in the tubing of the administration set (e.g., the patient has rolled onto part of the tubing and, as a result, pinched off the flow), or it may be caused by an empty reservoir condition. The former cause is not immediately dangerous and the flow controller need not shut down operation of the administration set. But, the latter cause is immediately dangerous since air may be received by the administration set if operation is not quickly stopped.
To the best of applicants' knowledge, prior flow controllers have either failed to distinguish between abnormal conditions or have provided system responses for detection of abnormal conditions which are unsatisfactory, especially in the case of abnormal flow caused by temporary occlusions.
Another problem arises in connection with prior flow controllers because they require manual inputting of data to compensate for drop chambers of different sized orifices. As a result of this requirement, in order for the flow controller to accurately monitor total volume by counting drops formed by an orifice in the drop chamber, it has been necessary to rely upon the correct manual positioning of switches usually located on the front control panel of the controller for identifying the size of the orifice to the controller. Because the operator of the flow controller must manually position switches, the controller is only as reliable as the operator.
A further problem associated with prior flow controllers is the conventional drop detector associated with each controller is limited in the degree of tilting of the drop chamber which may occur before the detector is unable to detect a falling drop. Because a drop detector is typically mounted around the drop chamber and not directly secured to a stationary base, the detector and chamber may be vertically angled. In fact, slight angles of a freehanging drop chamber and detector are common. Therefore, it is important that the drop detector dependably detects drops at angles which may be reasonably expected to occur during the use of the flow controller.